Question

我试图从我之前写过的网络API中获取一个列表。然后我将在Xamarin.Forms ListView中使用该列表。我的代码在这里：

public static class DataSource
{

    public static async Task<Restoran[]> GetRestoransAsync()
    {
        // ... Use HttpClient.
        using (HttpClient client = new HttpClient())
        using (HttpResponseMessage response = await client.GetAsync(page))
        using (HttpContent content = response.Content)
        {
            // ... Read the string.
            string result = await content.ReadAsStringAsync();

            var restorans = JsonConvert.DeserializeObject<Restoran[]>(result);
            return restorans;
        }

    }

}

我的ContentPage：

public class MenuPage : ContentPage
{
    ListView listView;
    List<Restoran> restorans = new List<Restoran>();
    async Task LoadRestorans()
    {
        restorans = (await DataSource.GetRestoransAsync()).ToList();
    }

    public MenuPage(string masa)
    {
        var loadData = LoadRestorans();
        loadData.Wait();
        listView = new ListView(ListViewCachingStrategy.RecycleElement)
        {
        ItemsSource = restorans,
            ItemTemplate = new DataTemplate(() => {
                var nativeCell = new CustomCell();
                return nativeCell;
            })
        };
    }
}

但是当我调试这段代码时，＆＃34; LoadRestorans（）＆＃34;方法在＆＃34; restorans＆＃34;的初始化之前调用。名单。我想我不了解异步方法的心态。我该怎么办？

Answer 1

您有两种选择。

使用异步工厂方法

创建页面

public class MenuPage : ContentPage {
    ListView listView;
    List<Restoran> restorans = new List<Restoran>();

    private async Task LoadRestoransAsync() {
        restorans = (await DataSource.GetRestoransAsync()).ToList();
        listView = new ListView(ListViewCachingStrategy.RecycleElement) {
            ItemsSource = restorans,
            ItemTemplate = new DataTemplate(() => {
                var nativeCell = new CustomCell();
                return nativeCell;
            })
        };
    }

    public MenuPage(string masa) {
        //...        
    }

    public static async Task<MenuPage> CreateMenuPageAsync(string masa) {
        var page = new MenuPage(masa);
        await page.LoadRestoransAsync();
        return pagel
    }
}

然后在其他异步事件处理程序中使用它

var page = await MenuPage.CreateMenuPageAsync("<masa here>");

或

在OnAppearing事件中执行此操作。

订阅页面/视图的Appearing事件

protected override void OnAppearing() { this.Appearing += Page_Appearing; }

并在实际的偶数处理程序上调用异步代码

private async void Page_Appearing(object sender, EventArgs e) { //...call async code here //unsubscribing from the event this.Appearing -= Page_Appearing; }

完整的课程看起来像这样

public class MenuPage : ContentPage { ListView listView; List<Restoran> restorans = new List<Restoran>(); private async Task LoadRestoransAsync() { restorans = (await DataSource.GetRestoransAsync()).ToList(); listView = new ListView(ListViewCachingStrategy.RecycleElement) { ItemsSource = restorans, ItemTemplate = new DataTemplate(() => { var nativeCell = new CustomCell(); return nativeCell; }) }; } public MenuPage(string masa) { //... } protected override void OnAppearing() { this.Appearing += Page_Appearing; } private async void Page_Appearing(object sender, EventArgs e) { //...call async code here await LoadRestoransAsync(); //unsubscribing from the event this.Appearing -= Page_Appearing; } }

之前发生的事情是通过调用.Wait()，这是一个阻塞调用，该类混合了异步和阻塞调用（如.Result和.Wait()），这可能导致死锁。因此，当您尝试测试时，为什么您无法通过该方法。

Answer 2

在restorans之前调用，因为在初始化属性之前调用了构造函数MenuPage。

我建议

public class MenuPage : ContentPage
{
    ListView listView;
    List<Restoran> restorans;
    async Task<Restoran[]> LoadRestorans()
    {
        return await DataSource.GetRestoransAsync();
    }

    public MenuPage(string masa)
    {
        this.restorans = LoadRestorans().GetAwaiter().GetResult().ToList();
        listView = new ListView(ListViewCachingStrategy.RecycleElement)
        {
        ItemsSource = this.restorans,
            ItemTemplate = new DataTemplate(() => {
                var nativeCell = new CustomCell();
                return nativeCell;
            })
        };
    }
}

注意：我没有编译这段代码，但根据我的理解，这应该是正确的代码。

HttpClient使用异步/等待操作获取列表

2 个答案: